This invention relates to a temperature-compensated fluid pressure indicator. A principal intended use of the invention is as a state-of-charge indicator for fire suppression bottles. Such bottles are commonly used in military land vehicles (tanks, personnel carriers, etc.) or aircraft to suppress explosive-type fires or slow growth fires.
Explosive type fires can occur in military vehicles when an enemy projectile ruptures the fuel tank on board the vehicle. The fire suppression bottle(s) within the vehicle contains a quantity of liquid fire extinguishant (e.g., three pounds) under relatively high pressure (e.g., 750 p.s.i.); the liquid extinguishant is commonly Halon-1301 (bromotrifluoromethane). The bottle is equipped with a fast-acting solenoid valve or explosive squib-actuated valve that is electrically connected to an electrical power supply controlled by a fast-acting optical detector responsive to generation of flame in or near the vehicle's fuel tank. The fire-suppression system is designed so that from the onset of the flame, the entire contents of the bottle are automatically discharged within about 100 miliseconds. No human actuation effort is required.
Military vehicles are also commonly equipped with portable manually-operated fire suppression bottle units for handling slow growth fires. Such portable bottle units are (or may be) charged with Halon-1301 (by the same procedures used to charge the bottles in the automatic systems).
Moderately fast-acting fires can occur in the engine compartments of vehicles due to engine malfunction. The described automatic bott1e systems can be used to suppress such fires. Manually-operated bottle units are not usually effective on engine compartment fires because it is difficult to gain access to the compartment in sufficient time.
Slow growth fires can be generated in military vehicles due to fuel/oil spills, electrical spark generation, inadvertent use of matches, enemy action, etc. Such fires can be suppressed with manual hand-held fire extinguisher bottles controlled by manual discharge valves. As noted above, such bottles can be charged with Halon-1301 under pressure, using the same charging procedures as are used to charge the automatic bottles.
A preferred fire extinguishant is Halon-1301 (bromotrifluoromethane); the usual pressurizing agent is nitrogen. In most cases, the bottle is a single chamber bottle having a wall thickness sufficient to withstand expected internal pressures on the order of 700-800 p.s.i., at 70.degree. F., and substantially higher pressures at temperatures approaching 160.degree. F. The Halon-1301 may be charged into the bottle on a weight bases (e.g., three pounds, five pounds, fourteen pounds, etc., depending on the bottle size); thereafter the pressurizing agent may be introduced from a high pressure source into the bottle until the internal bottle pressure is at a satisfactory level, e.g., 750 p.s.i. During the pressurizing process, it may be necessary to agitate the bottle to promote mixing of the nitrogen and Halon-1301; otherwise some of the nitrogen may undergo delayed dissolving in the Halon, thereby reducing the effective bottle pressure during the standby period (prior to the instant when the bottle system is used to suppress a fire).
Military vehicles using these bottle systems may at times be subjected to relatively high temperatures at or near 160.degree. F. (e.g., in desert atmospheres); at other times such vehicles may be subjected to relatively low temperatures at or near minus 65.degree. F. (e.g., in arctic temperatures). The temperature extremes can cause significant pressure changes within the bottles. The bottles are usually provided with pressure-relief valves designed to vent excess pressure out of the bottles. Escape of the Halon can also take place through seals and other minute openings in the bottle walls and/or valve joints. The Halon-1301 is a vaporizable liquid having a relatively low boiling point such that when excess pressure is vented from the bottle, some of the pressurizing gas escapes along with the Halon.
In a typical bottle system containing seven pounds of Halon, the weight of nitrogen would be about 0.28 pounds. The Halon partial pressure at 70.degree. F. would be about 188 p.s.i., and the nitrogen partial pressure would be about 562 p.s.i.; total system pressure would be 750 p.s.i. (at 70.degree. F.).
Loss of Halon (or nitrogen), or initial undercharging of Halon (or nitrogen) into the bottle, is disadvantageous in any bottle system (automatic or manual). The loss of Halon is disadvantageous in that less Halon is available for fire suppression purposes. The Halon loss means potentially inadequate coverage of the flame and/or inadequate pressure to achieve satisfactory discharge of the extinguishant within the desired time interval (e.g., 100 miliseconds). Lack of nitrogen in the bottle translates into an unsatisfactory discharge pressure.
The present invention is directed to a pressure indicator designed to detect possible loss or non-presence of fire extinguishant (e.g., Halon-1301) and pressurizing agent from a fire suppressant bottle during the standby period after initial charging of the bottle. The pressure indicator is constructed so that the effects of ambient temperature change are taken into account, i.e., compensated. The principal aim of the invention is to provide a device that indicates the state-of-charge of the associated bottle, i.e., the mass of Halon and pressurizing agent within the bottle. The term "temperature compensation" is here used to convey the idea that the indicator will automatically subtract from the pressure reading any pressure increase due to temperature increase, or add to the pressure reading any pressure decrease due to temperature decrease taking place after the initial bottle-charging process (usually at or about 70.degree. F.). The output or final reading of the indicator device will be related to the mass of pressurizing gas and liquid extinguishant in the bottle, i.e., the state-of-charge of the bottle, and its ability to suppress a fire.
The present invention may be considered an improvement or variant on a state-of-charge indicator disclosed in U.S. Pat. No. 3,946,175 issued to A. N. Sitabkhan on Mar. 23, 1976. That patent shows an indicator wherein temperature-compensation is provided by the use of a small reservoir 6 containing a fluid that has the same pressure-temperature response as the fluid charged into the bottle. A bellows 32 has its interior exposed to the pressure existing within the bottle; the external space surrounding the bellows communicates with the aforementioned reservoir 6, such that motion of the bellows is related to the pressure differential across the bellows, i.e., pressure in the space within the bellows versus the pressure in the space surrounding the bellows and the combined pre-loads and spring rates of the bellows and spring 42.
In the device disclosed in U.S. Pat. No. 3,946,175, the effect of temperature on the bellows is at least partially compensated because both atmospheres (within and outside the bellows) are exposed to a common temperature. In this sense, the device of U.S. Pat. No. 3,946,175 corresponds to my proposed device. However my invention overcomes an "unbalanced-pressure" problem that I believe exists with the device disclosed in the referenced patent.
In the Sitabkhan patent any unbalanced bellows condition is aggravated by the presence of spring 42 which is designed with an inside coil diameter that prevents bellows expansion that could damage the bellows 32. It is considered poor practice to guide a spring on anything except a smooth surface. The bellows certainly does not meet this criteria. The Sitabkhan device is believed to provide only a partial (imperfect) temperature-compensating effect.
The structural arrangement proposed in the Sitabkhan patent is also believed to be disadvantageous in the sense that the indicator mechanism is mounted within the fire suppressant bottle (tank). With such a location the mechanism tends to be rather large because the switch or mechanical part of the mechanism is at least partially outside the bottle, whereas the fluid pressure part of the mechanism is within the bottle. The mounting hole (aperture) in the bottle side wall forms a potential leakage path. The bottle wall is curvilinear, such that it is difficult to form flat seal surfaces thereon. The assembly tends to be rather complicated.
The arrangement proposed in the Sitabkhan patent includes a sealed reed switch 54 mounted within a stationary hollow post 48 having a sealed connection with the wall of the fire suppressant bottle. I propose an arrangement that obviates the need for special sealing mechanisms of the type contemplated in the Sitabkhan patent.
My proposed indicator system is believed to at least partially overcome problems inherent in the system of U.S. Pat. No. 3,946,175. I propose to use two similarly-constructed bellows arranged in axial alignment with one another; one bellows has its interior space communicating with the bottle interior, while the other bellows is a sealed bellows whose interior space is charged with a pressurized vaporizing liquid having the same pressure-temperature characteristic as that of the bottle charge. The use of two bellows would, it is believed, overcome sealing problems and problems relating to erratic coil spring forces inherent in the device of U.S. Pat. No. 3,946,175. I propose an indicator device that is self-contained and mountable externally of the associated fire suppressant bottle, no machining or other treatment of the bottle is required to accommodate the indicator device thereon.